1. FIELD OF THE INVENTION
This invention relates to the field of microphones and in particular to a microphone having a flat frequency response over a selected frequency range.
2. BACKGROUND ART
In certain audio applications, it is desired to provide a sensing microphone to monitor audio output over a specified frequency range. In such applications, it is desired that the frequency response of the microphone over the frequency range be substantially "flat". That is, the amplitude of the output signal generated by the microphone should be substantially constant regardless of the frequency of the input signal. In the prior art, it has been difficult to provide a microphone with such flat frequency response. One problem associated with the prior art microphones is the resonant frequency of various elements of the microphone. For example, at the resonant frequency of a microphone diaphragm, the frequency response spikes to a high amplitude. A prior art attempt to solve this problem is to use a relatively flexible diaphragm which is acoustically dampened to compensate for this resonant frequency.
Another prior art attempt to provide a flat frequency response is to utilize post sampling filtering to compensate for peaks and valleys in the frequency response output curve of the microphone.
One type of prior art microphone is a condenser microphone which utilizes a stretched membrane diaphragm. However, multiple resonances can occur within the stretched membrane leading to variable frequency response characteristics.
Prior art microphones utilize a diaphragm which is flexible and low in mass. Such a diaphragm is chosen to minimize the Q value so that compensation at the resonant frequency can be achieved. A certain degree of flexibility is also required in the diaphragm for sensitivity in the desired frequency range and amplitude range.
One prior art microphone is illustrated in Imay, U.S. Pat. No. 4,559,418. Imay is directed to a ceramic microphone having a diaphragm for receiving soundwaves and a thin ceramic plate coupled to the diaphragm for transducing the soundwaves to electric signals. A number of N divided electrodes are provided where N is an integer greater than 2 to act as serially-connected capacitors to multiply the voltage of the microphone output by a factor of N.
Another prior art microphone is described in Dunn, U.S. Pat. No. 4,431,873. Dunn relates to an omni-directional acoustic sensor with an air-backed diaphragm, with a piezo electric disk attached to each side of the diaphragm.
U.S. Pat. No. 4,734,611 to Granz relates to an ultrasonic sensor in which a polymer foil is piezo-electrically activated in a partial section. The connecting electrodes are spatially separated from the activated section, resulting in the ability to measure high pressure amplitude shock waves. The piezo section of Granz is isolated from a substrate or electrode.
A pipe-measuring device comprising a transducer having a flexible piezo electric layer as a sensor element is described in Ingle, U.S. Pat. No. 4,737,676. A piezo layer is applied directly onto a metal foil layer. An electrode is then coupled to the piezo layer and the device is used to measure mechanical movement of pipes.
None of these prior art microphones address the problem of response variation at the resonant frequency of microphone components.
Therefore, it is an object of the present invention to provide a microphone having a substantially uniform response over a desired range of frequencies.
It is another object of the present invention to provide a microphone in which the effects of the resonant frequency of microphone components is eliminated in the desired frequency range.